As integrated circuits become more complex and include more and more devices, it becomes necessary to develop new fabrication techniques which minimize the overall size of the circuit. One area of circuit fabrication which traditionally requires large quantities of substrate area is device isolation. Typically, an oxidation resistant material is patterned on the surface of the substrate and the substrate is oxidized to form large oxide isolation regions. This process is commonly referred to as localized oxidation of silicon or LOCOS. Because it is difficult to control the oxidation process with precision, an excessive amount of unwanted lateral oxidation growth often occurs. The lateral growth of the oxide isolation region consumes portions of the substrate which could otherwise be used for active device fabrication.
To avoid the lateral growth problem associated with the LOCOS process, new techniques have been developed which form isolatin regions in vertical recesses in the substrate. The process, commonly referred to as trench isolation, includes the formation of a dielectric body in a trench structure etched into the substrate. The dielectric body is formed by either an oxidation process, or a deposition and etching process. Since the dielectric material is confined to the boundaries of the trench, lateral consumption of the substrate substantially reduced.
While the trench isolation process offers a solution to the lateral growth problem encountered with the LOCOS process, important limitations remain. The trench isolation process necessarily requires the formation of trench in the substrate. Typically, the trench is formed by defining a photolithographic pattern on the substrate followed by an etching process. The trench can only be as small as the photolithographic definition capability will allow. As a result, the dimensions of the isolation regions are limited to the photolithographic capability of the fabrication process. Further problems can be encountered during the etching process. For example, if the etch process attacks the photoresist used to form the trench pattern on the substrate surface, the trench width will be undesirably enlarged. Unwanted enlargement can also occur if the etch process undercuts the photoresist and etches away portions of the substrate underlying the edge of the photoresist pattern. Accordingly, further development of isolation process technology is required to reliably form isolation regions which minimize the amount of substrate area necessary for their formation.